Certain entities of a telecommunication network are found to be critical entities expected to be fault tolerant since a failure in such entities may cause an important network failure and a huge amount of subscribers of such network not being able to communicate. In particular, where the telecommunication network is an IP Multimedia Subsystem (hereinafter IMS), a Serving Call Session Control Function server (hereinafter S-CSCF) serving a number of IMS subscribers is found to be one of said entities whose failure might make the IMS reach abnormal processing conditions so that the number of IMS subscribers served in said S-CSCF cannot properly make use of services or cannot even make calls. In this respect, most or all failures are accompanied by a reset of the failing entity and a sort of restart once the failure has been solved, likely by reloading permanent stable data if inconsistent data were found to be a reason for the failure. Moreover, not only the prevention of a failure in any critical entity is an issue but also other situations causing the critical entity to become out of service, such as a software or hardware update which cannot enter into operation without producing a restart of the critical entity concerned.
Regarding the abnormal processing conditions that an IMS network may reach after a S-CSCF restart, an interested reader has to take into account the different sequence of actions that may take place for a given subscriber simultaneously or immediately after the SCSCF restart, how such actions are said to be treated in accordance with the 3GPP Technical Specification 23.228 v.7.8.0 and v.8.1.0, and how such actions concern the S-CSCF. In particular, a given subscriber may register in the IMS in accordance with this 3GPP Technical Specification, and being assigned a particular S-CSCF selected for serving the given subscriber. This S-CSCF is assigned in a Home Subscriber Server (hereinafter HSS), which holds subscriber data for the IMS subscribers, for serving the given subscriber and the S-CSCF receives from the HSS a subscriber profile with subscriber data required for serving the given subscriber. An abnormal situation may occur where a first S-CSCF, namely the S-CSCF-1 depicted in FIG. 1, is presently serving the given subscriber and is thus assigned in the HSS, and where said S-CSCF-1 suffers a reset to restart again after having solved a fault situation. In this situation, the S-CSCF-1 is still assigned at the HSS for serving the given subscriber but the S-CSCF might have lost or not trust the subscriber data for the given subscriber.
FIG. 1 illustrates an exemplary abnormal situation that may further occur where an invitation to communicate addressing the given subscriber is received at an Interrogating Call Session Control Function server (hereinafter I-CSCF) from another IMS network. Then, the I-CSCF interrogates the HSS asking for a S-CSCF currently serving the given subscriber. As already commented above, the S-CSCF-1 is assigned in the HSS for serving the given subscriber, so that the HSS responds with an identifier of said S-CSCF-1. Before the I-CSCF is able to forward the invitation to communicate with the given subscriber served by the S-CSCF-1, said S-CSCF-1 suffers a restart and, as a consequence of which, the previously operative subscriber data at the S-CSCF-1 are lost or found to be not trustable. When the invitation forwarded from the I-CSCF to communicate with the given subscriber is received at the S-CSCF-1, the given subscriber is assumed unknown to the S-CSCF-1, and a confirmation of being serving the current subscriber is submitted towards the HSS to implicitly request subscriber data for the given subscriber. The HSS then returns a subscriber profile with necessary data for serving the given subscriber. However, since other data obtained during the original registration of the given subscriber were lost, the S-CSCF-1 ignores data concerning the signalling path between the given subscriber and the S-CSCF-1 such as, for example, a Proxy Call Session Control Function server (hereinafter P-CSCF) through which the given subscriber had accessed the IMS. In this situation, the S-CSCF-1 can only take actions such as connecting a mailbox to the originating subscriber or the like, but cannot connect the originating subscriber with the terminating given subscriber.
Another abnormal situation, not illustrated in any drawing, might occur where a previously assigned S-CSCF-1 suffers a restart during a handover procedure to assign a new S-CSCF for serving the given subscriber. The new assigned S-CSCF may obtain the subscriber profile but it ignores data concerning the signalling path towards the given subscriber. Also in this situation, the S-CSCF-1 cannot connect the originating subscriber with the terminating given subscriber.
Nowadays, such failures in telecommunication networks can be prevented, or are at least minimized by having mated entities which may enter into operation to replace a failing entity or an entity to be upgraded, such as a redundant HSS provides for, whereby once the restart has taken place, the redundant HSS updates the restarted HSS with all updated data and then the operation mode is switched back to make the restarted HSS operative whilst the redundant HSS is switched to a non-operating mode. However, the redundancy is an optional configuration suitable for subscriber databases holding subscription data for a huge number of quasi-permanent subscribers and not affordable for call control servers such as the S-CSCF where the served subscribers are changing very often. Therefore, there is a need to provide a mechanism for recovery of the IMS where a S-CSCF has suffered a restart after a failure, a software upgrade or other reasons.